@misc{indiciaea0b9ac60ba05,
  title = {Solution of the heawood map-coloring problem.},
  author = {G Ringel and J W Youngs},
  year = {1968},
  doi = {10.1073/pnas.60.2.438},
  url = {https://pubmed.ncbi.nlm.nih.gov/16591648/},
  note = {Source identifier: 16591648}
}